From another aspect, the hierarchy problem, which is the vast discrepancy between the electroweak scale and the Planck scale In popular terms, gravity is mysteriously much weaker than any other forces., motivated some possible solutions by modifying the structure of spacetime itself. Specifically, there are some models that introduce one or more extra spatial dimension(s) to explain the hierarchy problem; see Table REF  for a summary While extra spatial dimensional models are popular, it is also interesting to note that there exist an extra temporal dimensional model called Two-Time physics proposed by Itzhak Bars [1]}, [2]}. In this model, there are 4 flat, non-compact spatial dimensions and 2 temporal dimensions. Interested readers can confer Ref. [3]} for an inflationary scenario in this model. . The basic idea is that gravity is much weaker than other forces because there are extra dimensions to leak gravity out. In other words, all the fields except gravity are confined on an effective \(3+1\)  dimensional spacetime called the brane, while in fact there exist a higher dimensional spacetime called the bulk. These proposals seem to be too speculative to be true, but in fact extra spatial dimensions naturally arise in a quantum gravity theory known as string theory [4]}. A familiar argument for why these extra dimensions have not yet popped out in experiments is that they are tiny compact dimensions (such as RS1 and ADD models), which means that we need a very high-energy scale and a very delicate instrument to detect, for example, a quick energy non-conservation process. An even more ambitious idea is that the extra dimensions are not small at all (such as RS2 and DGP models), but still they do not cause any significant effects that can be easily detectable in a lab; for example, the RS2 model predicts a modification to the Newton's gravitational law at small distance but there is a parameter space of the model that is still allowed within the experimental bound [5]}. In this paper, we shall work on the Randall-Sundrum type II model (RS2) [6]} as it is more interesting to study cosmological phenomenology in this model. Recently, it has even been shown that there can exist humanly traversable wormholes in this model [7]}.
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